1. Field of the Invention
The present invention relates to a Y/C separation circuit for separating a composite video signal into a luminance signal and a chrominance signal.
2. Description of Related Art
Y/C separation circuits are used for separating a composite video signal such as an NTSC signal or a PAL signal into a chrominance signal (C signal) and a luminance signal (Y signal). A Y/C separation method includes a frequency separation method, a two-dimensional Y/C separation method, and the like. Among those, the frequency separation method extracts from the composite video signal, a frequency band of a chrominance subcarrier obtained by modulating the chrominance signal, through a band-pass filter. The frequency separation method only requires the band-pass filter for separation, so the separation can be easily performed. As for a high-contrast image etc., a high-frequency component of the luminance signal passes through the band-pass filter, which would cause a contrast degradation and a cross-color artifact.
The two-dimensional Y/C separation method separates a chrominance signal from a luminance signal through a comb filter based on the relation that luminance signals of adjacent horizontal scan lines are in same phase, and chrominance subcarriers of adjacent lines are in opposite phase, insofar as a horizontal correlation is maintained. To be specific, when signals of adjacent lines correlate with each other, the signals of adjacent lines are subtracted from each other, so the luminance signals are cancelled out to extract the chrominance signal alone. Further, the extracted chrominance signal is subtracted from the original composite video signal to obtain the luminance signal.
As regards the NTSC signal, a chrominance subcarrier inverts its phase in one horizontal scanning period. As regards a PAL signal, a chrominance subcarrier inverts its phase in two horizontal scanning periods. Thus, adjacent lines in this specification imply lines ahead of and behind one horizontal scanning period for the NTSC signal, and imply lines ahead of and behind two horizontal scanning periods for the PAL signal.
In the two-dimensional Y/C separation method, the Y/C separation is carried out by use of a correlation (vertical correlation) between the signals of adjacent lines, so a luminance signal may be well separated from a chrominance signal if there is little change in luminance (luminance transition) and in color (color transition) between lines, and a vertical correlation is strong. However, as for an image having a diagonal stripe pattern in which a luminance between lines abruptly changes or an image involving a sharp color change between lines in a horizontal direction, a vertical correlation between lines is not so strong, so a chrominance signal cannot be well separated from a luminance signal with the two-dimensional Y/C separation method. As regards such an input signal, a chrominance signal may be well separated with the frequency separation method rather than with the two-dimensional Y/C separation method.
In this way, the optimum Y/C separation method varies depending on the correlation between lines of the video signal. Hence, there has been proposed hitherto a Y/C separation circuit (hereinafter referred to as adaptive Y/C separation circuit) in which a frequency separation method and a two-dimensional separation method can be selected according to an input signal (see Japanese Examined Patent Publication No. 60-47793, Japanese Unexamined Patent Publication No. 10-224816, and Japanese Unexamined Patent Publication No. 2000-175219, for example). Such adaptive Y/C separation circuits detect a correlation between adjacent two lines or adjacent two of three lines to select a filtering operation applied to the chrominance signal separation based on the detection result.
For example, the adaptive Y/C separation circuit disclosed in Japanese Examined Patent Publication No. 60-47793 determines the presence/absence of the horizontal correlation by calculating a difference in signal amplitude between signals in three lines, and judging whether the amplitude difference is a predetermined threshold or less. Besides, if the amplitude difference exceeds the threshold, the correlation (horizontal correlation) between signals of the same line is detected to determine whether the vertical correlation is severed due to a luminance transition or a color transition. Based upon the determination result of the vertical correlation and horizontal correlation, a filtering operation for separating a chrominance signal is selected.
As mentioned above, the conventional adaptive Y/C separation circuit not only compares signals of three lines in terms of amplitude and phase, upon the determination for selecting a filtering operation for separating a chrominance signal, but also detects a correlation based on a signal amplitude difference between signals of three lines, and detects a correlation based on the vertical correlation and horizontal correlation. Thus, the conventional adaptive Y/C separation circuit has a problem that operations for detecting a correlation are complicated.